The present invention relates generally to an adaptive attenuation circuit and, more particularly, to an adaptive attenuation circuit for adaptively attenuating an alternating differential voltage produced by a magnetic or variable reluctance sensor in response to rotation of a wheel, while maintaining noise immunity.
Inductive magnetic sensors are commonly employed for automotive applications and the like to provide timing signals which enable the determination of position and speed of a rotating wheel. For example, specific applications may include the determination of engine crankshaft position and speed (i.e., RPM) and the determination of wheel speed for anti-lock braking systems. Inductive magnetic sensors generally used for these types of applications are commonly referred to as variable reluctance sensors.
The variable reluctance sensor is generally located adjacent to a rotating wheel which typically has a plurality of circumferentially spaced slots formed therein. The sensor has an inductive magnetic pickup that is generally made-up of a pickup coil wound on a permanent magnetic core. As the wheel rotates relative to the pickup coil, an alternating voltage is generated in the pickup coil when the slots on the wheel travel past the sensor. The alternating voltage must then be correctly decoded to recognize high or positive voltage levels. The frequency of the alternating voltage is then determined to achieve rotational speed information about the wheel.
The alternating voltage that is produced with the variable reluctance sensor has peak voltages that generally vary in amplitude according to the rotational speed of the wheel. In a number of automotive applications, the amplitude of the peak voltage may vary from approximately 250 millivolts (mV) at low end speeds to over 250 volts (V) at higher rotating speeds. However, the sensor output is usually fed to a processing module or other control device that is designed to operate within a more limited voltage range. For instance, automotive processing modules are commonly designed with 5 volts CMOS transistors in order to accommodate size and power constraints. For a 5-volt processing module, the input signal may not exceed 5 volts in order to protect the circuitry. Therefore, in order to accommodate a 0 to 5 volt range, the sensor output voltage must be properly attenuated when necessary.
In the past, one problem that has remained with some approaches has involved the inability to achieve a limited input voltage without sacrificing noise immunity and accuracy. For example, one approach to limiting the voltage suggests clipping the sensor output voltage at the rails (i.e., ground and 5 volts) and then using the clipped voltage to obtain the frequency information. However, this voltage clipping approach is very susceptible to noise interference, especially at higher speeds where the noise of the signal can approach the 5 volt limit.
Another approach for attenuating the output voltage of a variable reluctance sensor is discussed in U.S. Pat. No. 5,144,233 issued to Christenson et al. and entitled xe2x80x9cCrankshaft Angular Position Voltage Developing Apparatus Having Adaptive Control and Diode Control.xe2x80x9d This approach uses a resistive divider network which is controlled by the forward voltage of a diode. The above-referenced approach has the variable reluctance sensor connected in a single-ended configuration with one end of the pickup coil connected to ground. In addition, the circuit is capable of swinging to the level of the battery. This single-ended approach operates such that when the input voltage becomes high enough in amplitude to forward bias the diode, a resistive path is established to create a resistor divider network that attenuates the input voltage. However, this approach does not teach the attenuation of a differential voltage, and may not provide sufficient attenuation control that is necessary to protect the 5-volt gates associated with the processing module.
Another approach uses an RC network to attenuate the input voltage. While conventional RC filtering approaches have provided suitable attenuation for low voltage attenuation, high voltage attenuation applications such as those associated with Direct Ignition System (DIS) have exhibited limited attenuation capability. That is, it is generally difficult to provide substantial voltage attenuation for high voltages and also more difficult to control the attenuation. In addition, conventional usage of the RC filter introduces the propensity of a phase shift between the inputs for a differential voltage, thereby potentially causing significant errors in zero-crossing position determination.
One approach that uses variable resistance to attenuate the input voltage is disclosed in U.S. Pat. No. 5,450,008, entitled xe2x80x9cAdaptive Loading Circuit for a Differential Input Magnetic Wheel Speed Sensor,xe2x80x9d issued to Good et al., and also disclosed in U.S. Pat. No. 5,510,706, entitled xe2x80x9cDifferential to Single-Ended Conversion Circuit for a Magnetic Wheel Speed Sensor,xe2x80x9d issued to Good, and U.S. Pat. No. 5,477,142, entitled xe2x80x9cVariable Reluctance Sensor Interface Using a Differential Input and Digital Adaptive Control,xe2x80x9d issued to Good et al. The aforementioned U.S. patents are hereby incorporated by reference. The approach disclosed in U.S. Pat. Nos. 5,450,008, 5,510,706, and 5,477,142 discuss an adaptive loading circuit that provides variable attenuation by switching binary weighted resistors between input nodes. Included in this approach are multiple resistors and switch pairs which provide voltage attenuation. This approach differentially attenuates the voltage input and is capable of providing sufficient attenuation on high input amplitudes for a 5-volt interface circuit. However, a minimum of three resistors are required for each of the differential inputs. In addition, the switch that is employed is designed to be large so that impedance of the voltage divider is dominated by the resistor, which makes for a large area. Further, the input resistance must vary from one (1) to sixty-four (64) as the switch area varies from sixty-four (64) to one (1). This range of values can make it difficult to match various attenuation stages and also adds to increased circuit area.
A more recent approach to providing variable attenuation for a differential variable reluctance sensor has a current mode as disclosed in U.S. Pat. No. 6,040,692, entitled xe2x80x9cVariable Attenuation Circuit for a Differential Variable Reluctance Sensor Using Current Mode,xe2x80x9d which is hereby incorporated by reference. According to the aforementioned approach, a variable attenuation circuit and a rectifier and differential to single-ended conversion circuit operate in a current mode to attenuate a differential input voltage. The variable attenuation circuit receives an input differential voltage from a magnetic sensor and converts the differential voltage to current. Current sourcing circuits provide variable current attenuation by switching in and out transistor-based current sourcing branches. The rectifier and differential to single-ended conversion circuit converts the variable attenuated currents to a voltage output. The output of the rectifier circuit is applied to an adaptive threshold circuit. Unlike some prior approaches, the circuit maintains constant input impedance even when attenuation is applied.
The current mode attenuation approach disclosed in U.S. Pat. No. 6,040,692 offers a number of advantages over previous approaches including the presence of a constant input impedance. However, because the input voltage signal has been converted to a current signal and then is subsequently processed as a voltage signal, there may be an error in the absolute amplitude of the rectified voltage signal which is a function of temperature of the resistors on the application specific integrated circuit (ASIC). As the ASIC resistors increase in temperature, the amplitude of the amplified signal correspondingly increases which directly affects the low amplitude voltage that is detected by the circuit. Because the integrated resistors may vary due to process and temperature variations, the low amplitude detection circuit can vary by as much as plus or minus thirty-three percent (xc2x133%) from the desired level.
Accordingly, it is therefore desirable to provide for enhanced adaptive attenuation of an alternating differential voltage generated by a magnetic sensor in response to the rotation of a wheel. It is further desirable to provide for an accurate transmission speed sensor interface that is compatible with various sensors and provides an optimized signal-to-noise ratio in the system, without requiring different input configurations. Further, it is desirable to provide for an attenuation circuit that is compatible with input filtering regardless of the circuit attenuation and that provides a detection circuit with enhanced accuracy to ensure that noise associated with low amplitude input signals is removed.
In accordance with the teachings of the present invention, a variable attenuation circuit is provided for adaptively attenuating an alternating differential voltage that is produced by a magnetic sensor in response to rotation of a wheel. According to one aspect of the invention, the attenuation circuit includes an input circuit for receiving an input alternating differential voltage from a magnetic sensor via first and second input lines. The input circuit includes an RC filter coupled to the first and second input lines for attenuating high frequency signals. The attenuation circuit also includes a voltage to current converter circuit for converting the input alternating differential voltage to a differential current signal. The voltage to current converter circuit has an input coupled to the input current, and the input has a constant input impedance. The attenuation circuit further includes variable attenuation circuitry for selectively attenuating the differential current signal, a current to voltage conversion circuit for converting the attenuated differential current to a voltage signal, and an output for providing the output voltage within a preferred voltage range.
According to another aspect of the invention, the attenuation circuit includes an input circuit for receiving an input alternating differential voltage from a magnetic sensor via first and second input lines, a voltage to current converter circuit for converting the input alternating differential voltage to a differential current signal, variable attenuation circuitry for selectively attenuating the differential current signal, and a current to voltage converter circuit for converting the attenuated differential current to a voltage signal. The current to voltage converter circuit has an internal resistance. The attenuation circuit also includes a comparator for comparing the voltage signal to a threshold, and an initial threshold circuit for generating an initial threshold voltage for comparison by the comparator. The initial threshold circuit generates an initial threshold voltage that varies proportional to internal resistors of the current to voltage converter circuit.